All successful aircraft designs must address numerous design parameters in order to provide the necessary lift and thrust to overcome the associated drag and weight of an aircraft so as to fly. For example, common design parameters which affect the aerodynamic efficiency or performance of an aircraft include the aspect ratio (AR), weight, the lift coefficient, the drag coefficient and stability factors (typically indicated by yawing, pitching, and rolling moments) both at high speed and low speed conditions.
The central challenge of aircraft design is to maximize the amount of lift generated by an aircraft for the amount of associated drag, i.e., maximize the lift to drag (L/D) ratio. Of course the shape of the aircraft's wings also impacts the distribution of aerodynamic pressure or force over the surface of the wings which, in turn, affects the rotational and structural capabilities and limitations of the resulting aircraft.
As set forth below, the AR is generally thought to be one of the more important aircraft design parameters and can be computed as follows: EQU AR=(span).sup.2 /area
wherein "span" is the distance from one wingtip to the other wingtip and "area" is the surface area of the wings. Higher aspect ratio wings are generally thought to be more aerodynamically efficient because they provide a better L/D ratio.
Another aircraft design parameter is the sweep angle of the wings which can be described as the angle between a line drawn a quarter of the way between the leading and trailing edge of a wing and the spanwise or lateral direction. Wings with lower sweep angles generally yield higher lift force components, while wings having greater sweep angles are typically more desirable at flight speeds close to the speed of sound in order delay the onset of undesirable compressibility effects.
In order to address the various aircraft design parameters, a number of different types of aircraft have been developed. In this regard, conventional aircraft can be generally divided into two categories--a cantilever wing type aircraft and a joined wing type aircraft. Prior joined wing aircraft configurations employ two sets of wings which are rigidly interconnected or joined. As a result, the joined wing structure is self-bracing. In contrast, a cantilever wing structure employs wings which have no self-bracing feature, but which, instead, extend laterally outward from the fuselage in a manner independent of the other wing. A cantilever wing structure must therefore have stronger supporting struts and, in some instances, requires thicker wings fabricated from stronger materials. In comparison to cantilever wing configurations, structurally speaking, joined wing aircraft can have lighter and stiffer wings. Further, joined wing aircraft can also have improved aerodynamic characteristics such as higher span-efficiency factors which result in lower induced drag.
While there are a number of prior patents describing joined wing aircraft, none of the patented aircraft have adequately addressed all of the various aircraft design parameters. For example, U.S. Pat. No. 5,046,684 issued to Wolkovitch entitled Airplane with Braced Wings and Pivoting Propulsion Devices describes an airplane having two sets of wings at different planar positions relative to each other. These offset wings sweep in opposite directions to meet and join at a tip area. The text of this reference is hereby incorporated as if recited in full herein.
In particular, the Wolkovitch '684 patent describes the use of a vertical tail to mount an aft wing higher than a forward wing, thus, forming a triangular braced wing configuration. The tail, while providing inherent stability to the aircraft, must be of fairly rugged construction in order to carry the entire aft wing loads as well as its own load. As a result, the tail would likely be sized thicker than the aerodynamically optimum tail, potentially resulting in a tail-heavy condition. Further, the increased drag introduced by this non-coplanar wing configuration also reduces the L/D ratio.
The triangular braced wing configuration proposed by the Wolkovitch '684 patent is also configured such that the large forward wing and aft wing dihedral creates a significant amount of wing area when viewed from the side of the aircraft. The dihedral angle is the angle between the plane of the wing and the horizontal plane as seen in the front view.
Notably, any side projected area of the wings forward of the center of gravity is directionally destabilizing, although it is at least partially counter-balanced by the side projected area of the aft wing which is stabilizing (like the vertical tail). Thus, the side projected area of both wings does not provide any aerodynamic benefit and would most likely add to the aircraft's drag without increasing its lift.
U.S. Pat. No. 4,365,773 also issued to Wolkovitch entitled Joined Wing Aircraft also describes a joined wing aircraft which employs a tail and a pair of wings. The first set of wings extend outwardly in opposite directions from a medial portion of the fuselage to join the ends of respective ones of a second set of wings which are mounted to the top of the tail and extend downwardly and forward therefrom. Again, the wings are positioned at different planar levels and require the tail to support the aft wings, thereby creating a heavier vertical tail and aft body. Thus, even though a variety of joined wing aircraft have been designed in an attempt to optimize one or more of the various aircraft design parameters, each of these joined wing aircraft designs still suffers from several deficiencies which limit the aircraft's performance capabilities.